A. Field of the Invention
The present invention relates to prosthetic knee devices which overcome certain limitations of existing knee prostheses, generally referred to as semi-constrained knee prostheses, and more specifically as posteriorly-stabilized knee prostheses. The present knee prosthesis provides for an expanded range of degrees of translational movement without substantially reducing the risk of inadvertent dislocation of the knee when implanted. The devices of the invention achieve the increased ranges of motion without requiring a substantial posterioral shifting of the points of contact between the femoral and tibial components. The prostheses of the invention also reduce the risk of breakdown of the underlying resectioned proximal tibia surface.
B. Description of the Related Art
When natural healing of a diseased or injured knee is not sufficient to return the affected knee to its pre-injury or pre-disease condition, a preferred alternative is knee replacement surgery where certain portions of the natural knee are replaced with implanted prostheses. There are numerous types of such prostheses. One general category of knee prosthesis is the semi-constrained knee prosthesis of which an example is the posteriorly-stabilized knee. When the cruciate ligament is removed during resectioning, a posteriorly-stabilized prosthesis must be used. These knees generally possess a femoral component with a recess capable of interacting with a protuberance (peg, post) from the surface of the tibial component in order to stabilize movement of the knee components. While such semi-constrained, posteriorly-stabilized knee prostheses have gained in acceptance in the medical community, certain limitations have been found with existing knees of this type.
In certain prosthetic knees of this type, there is substantial risk of unwanted contact between the top of the peg and the roof of the recess. In most instances, were such unwanted contact to occur, the full weight of the upper leg and body would bear down on a small and relatively insubstantial surface (the superiormost surface of the tibial peg) not designed to carry such loads. In certain existing knees, this is avoided by lowering the height of the tibial peg. However, lowering the tibial peg increases the risk of an inadvertent dislocation event. In other existing prostheses, voids are left in the roof of the recess to prevent contact of the recess roof and the top of the tibial peg.
In other designs of this general category, mechanisms are provided which upon flexing of the knee joint, cause the femoral component to move substantially posteriorly in relation to the tibial component. In certain instances this posterioral shift is required by a particular device, especially where the device is to be implanted in a resectioned knee which is not resectioned with an approximately 7.degree. posterioral slope in the proximal tibial bone surface. In any case, such camming mechanisms increase the degree of flexure possible in those knee prostheses in which they are found and such increase flexure is generally desirable. However, in those knees where camming causes the contact points between the femoral condyles as they are nested in the tibial concavities to shift posteriorly, the tibial component must be designed to fully support the load on the knee joint at progressively more posterior positions as the knee translates through its full range of flexure and extension. Otherwise, where the tibial component does not properly support the load, softer plastic components will exhibit wear causing ultimate failure of the device and breakdown of the underlying resectioned bone surface. Where significant wear of plastic components occurs, disease conditions (osteolysis) may arise as a result of interaction of plastic particles and bone tissue. It is highly preferred to have knee prostheses with at least that degree of flexure observed in the natural knee so long as such may be achieved without increased risk of dislocation and without damage to the underlying bone surfaces.
The natural knee is capable of at least several degrees of hyperextension beyond full extension without dislocating. In certain existing prosthetic knees, little if any provision is made for extension beyond normal full extension. In some instances, this lack of hyperextension ability is a result of the need in these designs to prevent dislocation of the knee joint before and at full extension. In some of these designs, the anterior surface of the tibial peg may be essentially vertical only obtusely angled in relation to the base plane of the tibial component, abruptly ending translation upon reaching full extension. It is highly preferred to allow for at least that degree of hyperextension observed in the natural knee so long as it may be achieved in the knee prosthesis without increased risk of dislocation.
The threat of destroying the resectioned bone surface underlying a tibial implant is of equal concern along the lateral portions of the implant. In particular, such threats to lateral surfaces of the underlying bone when natural knee motions other than translational motion occur, such a lateral angulation (sometimes referred to as varus or valgus motion) consisting of a lateral sliding or rocking type motion. If the tibial component of the knee prosthesis fails to provide sufficient lateral support for such motions, the plastic components may wear and the outer edges of the bone surface underlying the unsupported tibial component medial/lateral extremes may exhibit cantilevering degradation. It is highly preferred that a prosthetic knee allow for those ranges of lateral motions which are exhibited by the natural knee so long as the medial/lateral edges of the tibial component of the prosthesis adequately protect against breakdown of the plastic components or the underlying surface of the resectioned proximal tibia.
Knee prostheses are needed with expanded ranges of translational movement without increasing the risk of dislocation of the knee. Preferably, such prostheses will achieve greater ranges of both extension and flexure as those motions are exhibited by the natural knee, and will do so without requiring a substantial posterioral shifting of the points of contact between the femoral and tibial components. Where possible, such prostheses will also reduce the risk of breakdown of the plastic components or the underlying resectioned proximal tibia surface, medially, laterally, anteriorly and posteriorly.